1. Field of the Invention
This invention relates generally to an improved position control means for robotic handling systems and more particularly, to an improved system and method for transferring a substrate to a predetermined position in a processing chamber.
2. Description of the Prior Art
In the manufacture of integrated circuits, semiconductor substrates are loaded into various reaction and other chambers using automated equipment for processing. Equipment has been designed including a robot that can transfer a semiconductor substrate, such as a silicon wafer, from a cassette through a central transfer chamber and into one or more processing chambers about and connected to the transfer chamber, in which the robot is located. It is desirable to know the exact location of the semiconductor substrate relative to the processing chamber so that the substrate can be precisely positioned at an optimum location within the processing chamber to maximize the effectiveness of the processing onto the precise desired surface area of the substrate to be processed. Likewise, it is also desirable that the substrate positioning apparatus that is used as a reference point and upon which the substrate is transported be routinely calibrated so that positioning error is minimized, if not eliminated.
Currently, there are few known different methods and systems for locating the centerpoint of semiconductor substrates; they include those disclosed in: Spencer et al, U.S. Pat. No. 4,833,790, entitled METHOD AND SYSTEM FOR LOCATING AND POSITIONING CIRCULAR WORKPIECES, issued May 30, 1989; and, Cheng et al, U.S. Pat. No. 4,819,167, entitled SYSTEM AND METHOD FOR DETECTING THE CENTER OF AN INTEGRATED CIRCUIT WAFER, issued Apr. 4, 1989, and which patent is commonly assigned to Applied Materials, Inc., of Santa Clara, California, the Assignee of the present invention.
In Spencer et al, the method and system disclosed is of a "spindle" type whereby a wafer is transferred by shuttle to a spindle where it is incrementally rotated, the distances between the center of rotation to the periphery of the wafer is measured along a linear path by a sensor means, the wafer centerpoint offset is calculated by geometric analysis of the measurements, and the wafer centered on the spindle by the shuttle.
There are several disadvantages with the Spence: spindle type method and system. First, it is an entirely separate and distinct apparatus from the processing system. Having a separate centerfinding apparatus requires an additional step in the manufacturing process, adding cost and complexity and decreasing valuable throughput time. That is, the wafer cannot be directly unloaded by robot from the wafer storage cassette and transferred to a processing chamber without first being manipulated by the separate centerfinding apparatus. As a result, the Spencer et al spindle type system and method does not take advantage of the direct movement of the wafer as it is transferred from the wafer storage cassette to the processing chamber. In addition, the Spencer et al system shuttle may require periodic calibration by a separate calibration tool if the centerfinding method is to remain accurate. Furthermore, once the positioning method has been performed, the wafer is transferred to a separate wafer transport arm which may also require periodic calibration to maintain precision positioning of the wafer.
In Cheng et al, the system and method disclosed is of an "optical sensor array" type whereby a semiconductor wafer is moved along a linear path across an array of sensors positioned generally transverse to the linear path of the wafer support blade. This centerfinder method is performed upon the direct removal of the wafer from a storage cassette by a processing system robot and while en route to a processing chamber. The robot blade and peripheral edges of the wafer are detected separately by the optical sensors to calculate the coordinate center position of the wafer relative to the robot blade. An xy coordinate system is defined by the path (x) of movement of the robot arm/blade and the center line (y) of the optical sensors. The origin (0) of the y coordinate axis is defined by the position of the center sensor. The detection of the robot blade provides a reference point and origin (0,0) of the xy coordinate system from which to move the wafer to its destination point. The detection of points along the leading and trailing edges of the wafer provide points on the x axis generally parallel to the path of movement of the wafer and from which the centerpoint of the wafer can be geometrically determined. Once the wafer center position is geometrically determined, the wafer can be moved and positioned at the destination location.
The Cheng et al type centerfinding system overcomes the disadvantages of the Spencer et al type system in that a separate and distinct apparatus is not required to determine the centerpoint of the wafer. The centerpoint of the wafer is determined directly during movement of the wafer to its destination location. This is especially advantageous in a wafer processing system configuration where there exists a robot of a R-Theta type in a multiple chamber processing apparatus with a single loadlock chamber as shown in Cheng et al.
However, there are disadvantages to the Cheng centerfinding system. The first and foremost disadvantage is that the wafer must pass over the sensors in a linear path transverse to the position of the sensors which are positioned adjacent to the loadlock chamber. This means that the centerfinding operation can only take place when a wafer is being loaded or unloaded from the loadlock chamber adjacent to the position of the sensors. This is a distinct disadvantage when the processing system has multiple loadlock as well as multiple processing chambers. Each time a wafer is transported from one chamber to another, it must first be transferred back to the loadlock chamber adjacent the sensors so that the wafer can be passed through the sensors in the linear fashion shown in Cheng et al for the centerfinding method to be performed. As a result, the Cheng et al centerfinder system is ill-suited to the multiple chamber wafer processing system configuration in that it causes a decrease in valuable throughput time. If multiple sensor arrays are used, for example, if a sensor array is positioned adjacent to each loadlock and processing chamber, the increase in complexity and cost would render the configuration impractical. Furthermore, the algorithms used to geometrically determine the centerpoint of the wafer cannot be easily extended to include more than three sensors.
The second disadvantage of the Cheng et al centerfinder system is that once the wafer centerpoint is determined, the positioning of the wafer is relative to a reference point taken from the robot blade. This is a disadvantage in that the reference point is previously calibrated relative to the position of a "golden" wafer, hand centered on the blade. This manual intervention for calibrating the robot blade increases the chance of calibration error. In addition, the golden wafer used may have an undetected defect, or not be perfectly round, thereby increasing calibration error. Lastly, manual intervention using a separate and distance calibration tool such as a golden wafer is cumbersome and inefficient.
A third disadvantage of the Cheng et al centerfinder is that the sensor electronics are disposed within the system transfer chamber. The sensor electronics must be placed within the transfer chamber to reduce sensor error in the detection of the wafer. Being thus disposed, the components of the electronics can outgas, thereby contaminating the wafers. This is a further reason why a plurality of sensor arrays are impractical with this type of centerfinder system. The greater the number of sensor arrays, the greater the likelihood of contamination.
An additional disadvantage of the Cheng et al centerfinder is that the robot arm must be in an extended position for the wafer to be detected by the sensors. The extended stroke of the robot increases any calibration error and reduces the accuracy of the centerfinder operation.
Thus, a need has arisen for an improved system and method for determining the location of a semiconductor substrate with respect to a semiconductor substrate support which increases throughput time in a multiple loadlock and processing chamber system, includes sensor electronics disposed outside the transfer chamber for reduction of contaminates, is easily extendable to accommodate a plurality of sensor arrays, and wherein the location operation occurs when the robot arm is in a retracted position to minimize calibration error, if any. In addition, there is a need for an improved positioning system which can be calibrated without manual intervention or need for a separate and distinct calibration tool.